As a form of light indicating devices, light emitting diodes (LED's) provide several advantages over incandescent or gas discharge types of indicator lamps. Among these advantages are the small size, exceptionally long life, ruggedness and durability of the LED's and their consistent levels of light output over an extended period of time. Typically, the half-life of an LED, or that point in time at which initial light output is reduced by 50%, is theoretically projected to be approximately twenty years. Because of these advantages, LED's have been widely accepted in a large number of industrial and commercial product applications.
Since LED's typically operate at approximately two volts DC and require an external series current limiting resistor, applications are primarily found in electronic products which operate at lower AC and DC voltages. In higher AC voltage applications, the most common being 120 volts AC, the use of LED's is more difficult due to their lower operating and peak inverse voltage ratings. Specifically, a larger amount of voltage must be dropped across the series current limiting resistor resulting in a correspondingly larger amount of heat energy being generated in that resistor. Typically a rectifying diode is placed in parallel with each LED, and in higher AC voltage applications, a second diode is placed in series with the LED. The series diode is biased in the same direction as the LED, and the parallel diode is biased in the opposite direction of the LED. In 120 volt AC applications, both diodes are typically employed. The series connected diode rectifies the AC voltage and thus reduces the voltage drop across the series current limiting resistor, and the parallel connected diode prevents peak inverse voltages from exceeding those specified for the LED.
Even though the series connected diode rectifies half of a 120 volt AC sine wave, approximately 82 volts RMS must still be dropped across the series current limiting resistor. At this voltage, and at a typical average current through the LED of 15 milliampres, the RMS power or heat generated in the resistor will be approximately 1.867 watts. In a typical low DC voltage application, such as 5 volts DC, the voltage drop across the series current limiting resistor would be approximately three volts DC, and the power generated in the resistor would be approximately 0.045 watts at a comparable DC current of 15 milliampres. Thus the power generated in the series current limiting resistor in 120 volt AC applications is excessive when compared to the power generated at lower DC voltages. This excessive power or heat generation problem is compounded when several LED's are used in a particular 120 volt AC application and is further compounded if there is a limited and/or enclosed amount of space in which to locate the current limiting resistors required for each LED. In such cases, temperature rise can rapidly reach levels which can become component destructive.